The transmission of a digital signal in inverse multiplexing allows the use of several parallel transmission lines, particularly several parallel running optical fibers, with smaller bandwidth than would be required for the transmission of the digital signal. Here, the digital signal is split into several sub-signals, which in each case are transferred via one of the parallel running transmission lines from a transmit side to a reception side of the transmission lines. On the reception side, the sub-signals are assembled again to a digital reception signal, taking into consideration the correct sequence of the bits or bytes which were transmitted via different transmission lines. The transmission in inverse multiplexing thus allows the continuing use of existing parallel transmission lines (here, the term “parallel” strictly speaking only expresses that the individual transmission lines have a common, transmit-side starting point and a common, reception-side end point), even if a digital transmit signal requires a broader bandwidth than the bandwidth made available by each individual transmission line. Existing transmission units and receiving units of the individual transmission lines can optionally continue to be used, particularly those parts which carry out, for example, a signal conversion or a signal processing of the individual sub-signals.
For the optical signal transmission via an Optical Transport Network (OTN), the international standard ITU-T G.709/Y.1331 establishes how the transmission or the signal processing must occur at the transmit-side and reception-side interfaces.
The digital signal corresponding to a certain service, for example, an Optical-channel Transport Unit of Category 3 or 4 (OTU3 or OTU4 Service), is split for the transmission in inverse multiplexing into individual parts, the so-called Logical Lanes (LL), where then, in each case, a single or several LL's can be combined to a sub-signal. Each sub-signal generated in this way can then be transmitted via respective assigned transmission lines from the transmit-side end to the reception-side end. In the process, the running times of the individual sub-signals may differ greatly, so that, for assembling the sub-signals to a digital reception signal corresponding to the digital transmit signal, a synchronization of the sub-signals must take place in such a way that the individual blocks of bytes which in each case are assigned to an LL have the same sequence in the digital reception signal as in the digital transmit signal.
In contrast to the situation with a single digital signal transmitted via a single assigned transmission line, for the transmission of a signal in inverse multiplexing, the performance of the overall transmission line is influenced by parameters of several parallel transmission lines. Therefore, it is necessary or at least desirable here to determine parameters that characterize the transmission quality of the individual parallel transmission lines. These parameters can then be used to influence all or selected transmission lines, or the digital transmit signal in question, in each case separately in such a way that the result is an optimal performance via the transmission line in question or via the overall transmission line.
Naturally, during signal transmission in inverse multiplexing, it is also possible to determine the performance of the individual parallel transmission lines in the usual way by evaluating the individual sub-signal, for example, with regard to the bit error rate, before they are assembled again to the digital reception signal. However, this procedure is associated with corresponding high costs, particularly costs associated with the switching technology. Moreover, each signal in itself would have to be designed in such a way that recognition or correction of transmission errors is possible on the reception side.